The present invention relates generally to polyphenylene ether thermoplastics protected against discoloration during processing, and to methods for making said thermoplastic.
Discoloration is generally viewed as a detriment to the plastic, since it spoils the aesthetics, creates the appearance of non-uniformity, and is often associated by the customer with inferior properties, whether or not it is associated with actual degradation of the polymer. The art of compounding of plastics has led to a wide variety of stabilizers and discoloration preventatives which tend to differ from plastic to plastic.
In the foregoing and following discussions, the terms "preventing" or "prevention" or "preventative" is used in a relative sense rather than in an absolute sense, i.e. by prevention is meant suppression relative to an untreated control sample.
Vinyl polymers such as PVC are successfully prevented from yellowing by alkyltin compounds, barium-cadmium salts, and calcium-zinc salt mixtures. These types of stabilizers appear to prevent release of hydrogen chloride and prevent formation of long chains of conjugated double bonds which cause color. In polyolefins (which can degrade without a great deal of discoloration), the required stabilization is more directed to preventing loss of mechanical properties than to preventing discoloration; various phenols in combination with phosphites or organic sulfides are commonly used. In styrenic polymers such as ABS, it is also common practice to use tertiary-butylphenols plus phosphites; here again, much of the concern is protection against loss of mechanical properties, although prevention of discoloration is sometimes a consideration. In nylon 6,6, especially for fiber use, discoloration during thermal processing is a serious problem and discoloration preventatives such as phenylphosphinic acid are used. In polyester fiber and film, various phosphorus additives are used to prevent discoloration.
The development of the newer high-performance thermoplastics (the so-called "engineering thermoplastics") has imposed more stringent needs for improved means for preventing degradation, including that part of degradation which manifests itself as discoloration. The structures, mode of synthesis, and mode of degradation are different from the earlier plastics mentioned above. Most of these high-performance thermoplastics have heteroatoms (i.e. atoms other than carbon) in the backbone of the polymer, which makes their chemistry quite different from the chemistry of purely carbon-chain polymers. Moreover, the processing of these thermoplastics tends to be carried out at higher temperatures than the older lower-temperature thermoplastics. Discoloration is often a serious problem at the high temperatures used for mixing, molding, and extruding the engineering thermoplastics. The need is especially pressing for additives which can prevent discoloration in those engineering thermoplastics which are commonly processed above 450.degree. F. and frequently at or above 570.degree. F. Such thermoplastics include nylon 6, nylon 6,6, nylon 4,6, polyphenylene ether, polyamide-imides, polyarylates, polyarylene sulfones, aromatic polyesters, polyether-etherketones, polyether imides, polyether sulfones, polymethylpentene, polyphenylenesulfides, and blends thereof with each other and with other thermoplastics.
This requirement for prevention of discoloration during processing is unfortunately not satisfactorily met by polymer stabilizers which perform well in polymers processed at lower temperatures, such as polystyrene, polyethylene, polypropylene, ABS, diene elastomers, and the like. Some of the stabilizers described in the prior art for use in the engineering thermoplastics perform reasonably well in the lower end of the processing temperature range but decline badly in activity as the processing temperature reaches or exceeds 300.degree. C. (about 570.degree. F.). Moreover, while these stabilizers may have a beneficial effect in retarding loss of physical properties such as impact or tensile strength, they are often less effectual in preventing discoloration during thermal processing. Paradoxically, some stabilizers such as those containing aromatic amino structures can perform well as antioxidants while actually contributing to darkening of color.
This requirement for discoloration preventatives in the processing (hot mixing, molding, extrusion) of thermoplastics is also quite different from the requirement for stabilizers intended to protect the plastic during its service life, which for obvious reasons must be at temperatures lower than its processing temperature. Those stabilizers which are known for protection against atmospheric oxidation and photodegradation at the service temperatures of the plastic are generally found to be disappointing as high-temperature thermal processing stabilizers, and particularly disappointing in regard to prevention of discoloration during thermal processing. A further source of unpredictability lies in the structures of the polymers themselves. Different polymer structures have mechanistically different modes of discoloration, and the identity of the color-producing structures ("color bodies" or chromophores) is generally not known. From a knowledge of the "color bodies" in one kind of polymer, it is not possible to predict the factors affecting another kind of polymer.
A particularly little understood polymer class, in regard to thermal breakdown and discoloration, is the polyphenylene ethers and their blend compositions. It has been known since their discovery that these polymers discolor upon thermal processing. Up to now, this discoloration has had to be tolerated, or incompletely controlled, or masked to some degree by the use of a pigment, frequently titanium dioxide.
The patent literature shows many attempts to obtain high temperature processing stabilization of polyphenylene ethers by use of stabilizers known to be useful as processing stabilizers for other plastics (most frequently processed at lower temperatures) or by use of stabilizers known to be useful as long term stabilizers under service life conditions. In some instances, color improvement has been noted. Color improvement following the initial thermal processing of polyphenylene ethers and blend compositions thereof has been described using phosphites by Lee, U.S. Pat. No. 4,588,764 (May 13, 1986). Axelrod, U.S. Pat. No. 4,483,953 (Nov. 20, 1984), and Kinson, U.S. Pat. No. 4,405,739 (Sept. 20, 1983). Thermal degradation has also been associated by some inventigators with the presence of oxidatively-liable phenolic end groups and consequently there have been a number of patents directed towards masking these groups by chemical reactions; for example, Hay et al., U.S. Pat. No. 4,048,143 (Sept. 13, 1977). Such end capping entails a separate process step, is prone to incompleteness, and has not satisfactorily solved the problem of discoloration. A survey of many additives evaluated in polyphenylene ethers, and of the several theories regarding the degradation mechanism of the polyphenylene ethers, is given by Chandra, Prog. Poly. Science, Volume 8, 469-484 (1982).
Dispite this history of attempts to achieve color stabilization in polyphenylene ethers and blend compositions thereof, satisfactory results have been elusive and the resins of this class on the market remain undesirably colored. It has been particularly difficult to achieve good color stability towards the high end of the temperature range in which the polyphenylene ether blends are processed, for instance above about 570.degree. F. The manufacturer of a polyphenylene ether polymer must face the likelihood that some users will conduct their processing in this higher temperature range, and therefore it is desirable to have the polyphenylene ether formulated so that, as sold, it is capable of enduring these higher temperatures without discoloration.
It has not unexpectedly been found that compounds of the alpha-hydroxyketone class, which includes the benzoins, are discoloration preventatives for polyphenylene ethers and for blends containing polyphenylene ethers, since these alpha-hydroxyketones are themselves not exceptionally stable to heat; for example, benzoin was reported by Lachman, J. Am. Chem. Soc. 46, 717-718 (1924) to decompose at 300.degree. C. (572.degree. F.) to benzaldehyde and other products. It was also particularly surprising to find that at high temperatures, such as in the upper end of the processing range for polyphenylene ether blends, the lightness benefit given by the alpha-hydroxyketone is even greater than at temperatures in the lower end of the processing range.
It has been disclosed in European Patent Application No. 0176 811 (data laid open: 4/9/86) that certain benzoins are stabilizers for ABS, an acrylonitrile/styrene/butadiene graft copolymer. However, this European application indicates that the anti-discoloration action diminishes rapidly as the processing temperature is raised and becomes quite poor around 300.degree. C. (572.degree. F.), thus suggesting inapplicability to high-temperature-processed thermoplastics (see FIG. 1 in the cited application).